Since their discovery, the use of photoacid-generators, PAGs, has been widely adopted by the polymer industry in coatings, paints, anticorrosives, and electronics as described in Crivello, J. C. Journal of Polymer Science Part A: Polymer Chemistry (1999), 37, pp. 4241-4254.
The use PAGs in epoxide photoresists has been extensively studied for two-dimensional (2D) and three-dimensional (3D) lithographic patterning induced by one-photon absorption (1PA) and two-photon absorption (2PA) as described in Jeon, S.; Malyarchuk, V.; Rogers, J. A.; Wiederrecht, G. P. Optics Express (2006), 14, pp. 2300-2308 and Kim, E. K.; Ekerdt, J. G.; Willson, C. G. Journal of Vacuum Science & Technology B (2005), 23, pp. 1515-1520.
Two-photon absorption (2PA) has been reported for a number of applications, exploiting the fact that the 2PA probability is directly proportional to the square of the incident light intensity (dw/dt αI2) according to M. Goeppert-Mayer in Ann. Phys. (Paris) 1931, 9, 273; while one-photon absorption bears a linear relation to the incident light intensity, dw/dt αI). This intrinsic property of 2PA leads to 3D spatial localization, important in fields such as optical data storage, fluorescence microscopy, and 3D micro-fabrication.
There have been demonstrations of successful two-photon microfabrication using commercial PAGs, as reported by Belfield, K. et al. in J. Phys. Org. Chem. 200, 13, 837-849 and Belfield, K. et al in J. Am. Chem. Soc., 2000, 122, 1217-1218 even though the 2PA cross section of these initiators is low according to Schafer, K. et al. in J. Photochem. Photobiol., A. 2004, 162, 497-502. Two-photon 3D microfabrication with a novel PAG was reported to fabricate MEMs structures by Yu, T. et al. in Advanced Materials. 2003, 15, 517-521. To further advance a number of emerging technologies, there is a great need for photoacid generators (PAGs) with higher 2PA cross sections.
The synthesis of a class of photoacid generators such as, triarylsulfonium salts, by photolysis of the diphenyliodonium counterpart in the presence of a diphenylsulfide was originally reported by Crivello et al. in Abstr. Pap. Am. Chem. Soc., 1978, 176, 8-8 and J. Org. Chem. 1978, 43, 3055-3058.
Microwave-facilitated synthesis has been the subject of substantial interest over the last decade, as reported by Lidstrom, P. et al. in Tetrahedron. 2001, 57, 9225-9283 and Niilsson, P. et al. in Microwave Methods in Orsranic Synthesis 2006, vol. 266, 103-104. Seipel at al in J. Org. Chem. 2008, 73, 4291-4294 recently reported microwave-assisted reaction times are 80 times faster than conventional heating reaction times, and are potentially more energy-efficient than conventional heating. There are no known reports of the microwave-assisted synthesis of sulfonium salts and no knowledge that microwave assisted synthesis would provide the advantage of reducing reaction times, which is extremely useful and cost efficient.
Incorporated herein by reference ar two publications by the inventors: Ciceron O. Yanez et al., “Chatacterization of novel sulfonium photoacid generators and their microwave-assisted synthesis,” in Chemical Communications, 2009, 827-829, Jan. 7, 2009 and Kevin D. Belfield et al., “New Photosensitive Polymeric Materials for Two-Photon 3D WORM Optical Data Storage” presented at American Chemical Society Meeting, Philadelphia, Pa., August 2008, both published after the priority date of the U.S. Provisional Application Ser. No. 61/044,722 filed Apr. 14, 2008 from which the present invention claims the benefit of priority.
The present invention discloses a facile, microwave-assisted synthesis of triarylsulfonium salt photoacid generators (PAGs) suitable for use as two-photon absorbing (2PA) photoinitiators, in negative resists for photolihtography or 3-D microfabrication, or in optical data storage.